I can't understand why using laminar flow is using DNS method. Can laminar flow model account the turbulences in a non-laminar flow? I am also having a hard understand how laminar flow is solved in Fluent.

1)DNS isn't a method it simply means you don't use a turbulence model. You need incredibly dense grids though.

2)You need to look up a book for more info. As far as I know the laminar flow(which isn't a model) simply means viscous flow and so uses the Navier Stokes equations and the invscid flow doen't consider the viscosity and uses the Euler equations.

3)Laminar flow just solves the Navier Stokes equations without using a turbulence model. By using a very very fine grid you can directly compute the turbulence scales. RANS turbulence modeling just calculates a value for the turbulence that can be used by other function in Fluent.

I would advise you to read the fundamental(first few) chapters of any CFD book. Especially Computational Methods for Fluid Dynamics by Ferzier.

Please be aware I have no DNS experience so the above answers may need to be slightly modified.

Almost DNS modelling will be under following conditions: At usage LES of model in 3d case, In unsteady setting, With very mesh sizes of a mesh of a rated net. Such that the minimum size of turbulent pulsings (Vortexes) was more mesh width. All spectrum of turbulent pulsings (Vortexes) here is considered.

Almost DNS modelling will be under following conditions: At usage LES of model in 3d case, In unsteady setting, With very mesh sizes of a mesh of a rated net. Such that the minimum size of turbulent pulsings (Vortexes) was more mesh width. All spectrum of turbulent pulsings (Vortexes) here is considered.